avfilter_draw_slice() is already called in the end_frame() callback,
this avoids multiple calls. This is done by adding a null draw_slice()
callback.
In particular fix crash occurring with -vf transpose=3,hflip, fix trac
issue #371.
Improve consistency with libavcodec.
This breaks libavfilter API/ABI.
The non-sequential 2.1.0 -> 2.4.0 bump is due to the mess previously
done with the lavfi minor number.
